would be the case if it were made of platinum. It is also light, and can be easily fixed to a platinum wire. Among the important modifications of the tube are those which enable the operator to control the degree of vacuum in the tube. This is accomplished by sealing to the main tube an appendage containing certain chemicals which, on being heated, give off a small amount of vapor, and which take it up again on cooling. This modification is made necessary by the singular fact that after a Crooke's tube is submitted to an electrical discharge for some time the vacuum becomes more and more complete, and a higher and higher electro-motive force or pressure is needed to produce the discharge in the tube. It prefers in time to jump over the surface. Thus, at the very beginning of our use of the X rays we meet with a mystery. Where do
Fig. 1.—The evolution of the Crooke's tube.
the remaining particles of air go? It is surmised that they disappear in the platinum terminals.
The manufacture of the X-ray tubes tests technical skill and the patience of the experimenter more highly, perhaps, than the preparation of any apparatus used in science. Glass working is a difficult art, and requires an absolute devotion to it. There is only one metal known which will enable an electrical discharge to pass into and out of a rarefied space inclosed by glass. This is platinum. A wire of this metal can be sealed into glass so that no air can leak into an exhausted space around the joints. All electric lamps, so commonly used in electric lighting, have little wires of platinum at their bases, by means of which the electric current enters and leaves the bulb. The Crooke's tube is in principle an Edison lamp with the filament broken. The maker of Crooke's tubes should complete the making of the tube at one sitting, for
